Centralized process for the manufacture of a spunbonded fabric of thermobonded curled bicomponent fibers

ABSTRACT

A process is disclosed for the manufacture of a spunbonded fabric consisting of thermobonded curled bicomponent fibers where the bicomponent fibers consist of two plastic materials with different properties. The process according to the invention consist of three steps: the fibers spun from the two plastic materials are drawn off uncurled and stretched in a first step. In a second step the fibers are placed on a sieve belt and thermobonded. In a third step the bicomponent fibers of the obtained nonwoven fabric are finally curled by lengthwise and/or transverse stretching and heat treatment.

This application is a continuation of U.S. application Ser. No.09/269,556, filed on Mar. 30, 1999, now U.S. Pat No. 6,312,545 which wasa national stage filing under 35 U.S.C. §371 of InternationalApplication No. PCT/DE98/02083 filed on Jul. 21, 1998, whichInternational Application was not published by the International Bureauin English on Feb. 11, 1999.

The invention concerns a process for the manufacture of a spunbondedfabric of thermobonded curled bicomponent fibers, whereby thebicomponent fibers consist of two plastic materials with differentproperties.

Such a process is disclosed in EP 0 391 260 A1 where composite fibersplaced on a screen belt are curled and adhere to each other through theeffect of heat, i.e. heated air. In this process, cold air is blown frombelow through the sieve belt shortly before hot air is supplied to fluffthe nonwoven fabric. Such a nonwoven fabric is up to about 2.3 mm thick.Nonwovens created by such processes can be used in hygienic productssuch as diapers or sanitary napkins as an acquisition layer. For someapplications these nonwovens are too thin or have too hard a feel andare also not voluminous enough. A similar process is also disclosed inU.S. Pat. No. 5,382,400.

Common to these and other processes is the fact that the filaments arecurled solely by the treatment with heat. Thermobonding also occurswhile treating the filaments with heat, i.e. the individual fibers areadhered at their bonding points. The nonwoven fabric obtained in thismanner does not satisfy all requirements, especially when used forhygienic products, in particular they are frequently insufficientlyvoluminous.

The problem of the invention is to present a process of the initiallymentioned type that can create spunbonded fabrics from thermobondedcurled bicomponent fibers that are voluminous, and that can specificallyinfluence the properties of the nonwoven fabric with regard to strength,porosity, curling of the individual fibers and final weight. If needed,the watertightness of the nonwovens created by the process is to beinfluenced by the process.

In the invention process to manufacture spunbonded fabrics ofthermobonded, curled bicomponent fibers where the bicomponent fibersconsist of two plastic materials with different properties, the fibersspun from the two plastic materials are drawn off uncurled and stretchedin a first step, placed on a sieve belt and thermobonded in a secondstep, and the bicomponent fibers of the nonwoven fabric formed in thismanner are stretched lengthwise and/or transversely and curled by heattreatment in a third step.

It was surprisingly shown that the properties of the nonwoven fabriccould be directly influenced when the nonwoven fabric is stretchedlengthwise and transversely during heat treatment. The actual curling ofthe bicomponent fibers in the invention process is obtained duringrelaxing following lengthwise and/or transverse stretching. The rate ofcurling can be increased by the combined thermal and mechanicalinfluences on the fibers in the spunbonded fabric whereby the nonwovenfabric obtained in this way becomes more voluminous than the nonwovenfabrics obtained with prior art processes. With nonwoven fabrics createdby conventional processes, part of the curling and hence part of thevolume is lost during thermobonding and other heat treatments.

It was found to be advantageous to curl the nonwoven fabric bymechanical stretching only after thermobonding, thereby increasing thevolume of the nonwoven fabrics obtained in this way compared with thoseobtained by conventional processes.

A larger volume is particularly advantageous when the nonwoven fabricmade by the process according to the invention is used as an acquisitionlayer in diapers and sanitary napkins.

In one advantageous embodiment of the process, the latter is carried outwith bicomponent fibers of the side-by-side type. In another embodimentof the invention the process is carried out with bicomponent fibers withan eccentric core. By using solid or hollow bicomponent fibers of theside-by-side type and the type with an eccentric core, a softer productfeel can be obtained. This soft feel allows all nonwoven fabricspreviously used for hygiene to be replaced.

Another embodiment of the invention provides that the process be carriedout with segmented hollow bicomponent fibers where neighboring segmentsof each fiber consist of different materials. In another embodiment thefibers of the nonwoven fabric are stretched and heat-treated such thatthe fibers curl and fibrillate. In this, use was made of the knowledgethat the stretching especially of segmented bicomponent fibers beyond acertain degree of stretching will result in fibrillation, i.e. acontrolled tearing open or splitting open occurs. When the fibersstretched and fibrillated in this way are relaxed again, they are curledin addition to fibrillation.

A nonwoven fabric obtained in this manner possesses the same basisweight yet clearly lower porosity which makes the nonwoven fabric morewatertight.

In one advantageous embodiment of the invention the fibers are spun fromtwo polypropylene granulates. The MFI viscosity range of the firstpolypropylene granulate is 16 to 35 and the MFI viscosity range of thesecond polypropylene granulate is 8 to 10 points below the MFI viscosityrange of the first polypropylene granulate.

Alternatively, the fibers can be spun from two different polyolefins, ora polyolefin and PET.

The invention will now be further explained with reference to thedrawing, the description and patent claims. Shown in the drawings are:

FIG. 1: a cross-section of a diaper with an acquisition layermanufactured using a process according to the invention,

FIG. 2: a cross-section of a bicomponent fiber of the side-by-side type,

FIG. 3: a cross-section of a bicomponent fiber with an eccentric core,and

FIG. 4: a cross-section of a segmented hollow bicomponent fiber.

FIG. 1 shows an exemplary embodiment in the form of a diaper 10 thatuses a spunbonded fabric 12 created by the process according to theinvention. To create the spunbonded fabric 12 a hollow fiber 22 of theside-by-side type shown in FIG. 2 is spun with equipment known per se.The hollow side-by-side fiber 22 consist of two halves of a cylinderenvelope 24 and 26 of plastic materials with different properties.Accordingly, the cylinder envelope half 24 consists of polypropylenewith an MFI viscosity range of 16 to 35, whereas the cylinder envelopehalf 26 consists of polypropylene whose MFI viscosity range is 8 to 10points below that of the polypropylene of cylinder envelope half 24.

Numerous hollow fibers 22 obtained in this way are drawn off in such amanner that stretching them does not produce any curling yet. Then thehollow fibers 22 are placed on a sieve belt with equipment known per seand thermobonded. In a subsequent step, the spunbonded fabric 12 createdafter thermobonding is stretched in a device familiar from filmmanufacturing both in the direction of the machine and transversely.During this stretching process and the subsequent relaxation, curlingand hence fluffing of the nonwoven fabric 12 occurs in the individualhollow fibers 22 due to various stretching properties of the cylinderenvelope halves 24 and 26. It is particularly important for the nonwovenfabric to be simultaneously heat-treated.

The nonwoven fabric 12 obtained in this manner can now be used in thediaper 10 as an acquisition layer 16 placed over a core 14. Thisacquisition layer 16 serves to spatially separate the core 14 absorbingthe body fluid from the cover layer 20 contacting the organism. Thismeans that the acquisition layer 16 serves to prevent fluid that hascollected in the core 14 from recontacting the body surface of anorganism to be treated since the cover layer 20 must generally be keptvery porous so that it cannot be designed as a semi-permeable structure.Finally, the fluid stored in the core 14 is prevented from leaving by afilm 18 joined to the cover layer 20.

A spunbonded fabric manufactured according to the process of theinvention can be used as the material for the cover layer 20 for themanufacture of which bicomponent fibers 28 with a casing 30 and aneccentric core 32 shown in FIG. 3 are used. A spunbonded fabricmanufactured with such fibers 28 is suitable for use as a cover layer 20since it has a soft feel and is comfortable to the skin.

If a spunbonded fabric manufactured by the process of the invention issupposed to have sound waterproof properties, the fibers 34 portrayed inFIG. 4 are used to manufacture such. The cross-section of the fibers 34shown in FIG. 4 consists of individual segments 36 and 38. Neighboringsegments 36, 38 are made of different raw materials such aspolypropylene with different MFI viscosity ranges, polyolefins such asPP and PE in different phases, or a combination of PET with apolyolefin. When the fibers 34 are stretched they fibrillate; i.e. thefibers 34 split along the abutting surfaces of the segments 36, 38. Thisfibrillation of the fiber 34 clearly lowers the porosity while the basisweight of the nonwoven fabric remains the same resulting in an increasedwatertightness of the nonwoven fabric. This makes it possible to use anonwoven fabric manufactured in this manner as a cover material for thediaper 10 or for similar hygienic products.

What is claimed is:
 1. A process for the manufacture of a spunbondedfabric comprising the following steps: (a) drawing bicomponent fibersmelt spun from two different polypropylene granulates with a firstpolypropylene granulate having an MFI viscosity range of 16 to 35 and asecond polypropylene granulate having an MFI viscosity range which is 8to 10 points below the MFI viscosity range of the first polypropylenegranulate, (b) placing the bicomponent fibers on a sieve belt andthermobonding the bicomponent fibers to obtain a nonwoven fabric, (c)stretching the bicomponent fibers to be curled of the nonwoven fabriclengthwise and/or transversely and heating the bicomponent fibers duringthe stretching, and thereafter (d) relaxing the bicomponent fibers ofthe nonwoven fabric to obtain the spunbonded fabric with curledbicomponent fibers.
 2. A process according to claim 1, wherein thebicomponent fibers in step (a) are of a side-by-side type.
 3. A processaccording to claim 1, wherein the bicomponent fibers in step (a) have aneccentric core.
 4. A process according to claim 1, wherein thebicomponent fibers in step (a) are segmented and hollow with neighboringsegments of each fiber consisting of different materials.
 5. A processaccording to claim 4, wherein the bicomponent fibers of the nonwovenfabric are fibrillated in step (c) and curled in step (d).